Dear Mr Bayer and SQLAlchemy users,
We have an issue which probably counts as a feature request rather than a bug,
because it's sufficiently esoteric. We need to be able to use some Mappers (in
one Metadata) while other mappers (in a different Metadata) are not and cannot
be completely configured. This is because we fetch code (including Mapper
source files) from the database using SQLAlchemy (with a different
Metadata/declarative_base), and it's possible that we are in the middle of
loading Mappers when we need to fetch another source file from the database to
complete the configuration.
Here is an example that reproduces the problem that we see:
from sqlalchemy import Column, ForeignKey, Integer, Table, Text, create_engine,
event
from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy.orm import Mapper, configure_mappers, relationship, sessionmaker
from sqlalchemy.orm.mapper import _mapper_registry
_mapper_registry.clear()
FirstBase = declarative_base()
AnotherBase = declarative_base()
class Widget(FirstBase):
__tablename__ = 'widget'
id = Column(Integer, primary_key=True)
name = Column(Text)
class Animal(AnotherBase):
__tablename__ = 'mammal'
id = Column(Integer, primary_key=True)
name = Column(Text)
species = Column(Text)
@declared_attr
def __mapper_args__(cls):
return {
'polymorphic_on': cls.species,
'polymorphic_identity': cls.__name__,
}
# Register the first classes and create their Mappers:
configure_mappers()
# Simulate dynamic loading of an additional mapped class, which refers to one
that has not been loaded yet:
class Mammal(Animal):
# This mapping has an error, and therefore cannot be configured:
employer = relationship("Employer")
# These new classes should not be configured at this point:
unconfigured = [m for m in _mapper_registry if not m.configured]
assert len(unconfigured) == 1, str(unconfigured)
# Now try to query Widget, which is registered in FirstBase, which is
internally consistent:
engine = create_engine('sqlite://')
FirstBase.metadata.create_all(engine)
DBSession = sessionmaker(bind=engine)
session = DBSession(autocommit=True)
# We don't expect this query to fail because there is an error with a mapper in
AnotherBase, but it does:
widgets = session.query(Widget).all()
assert len(widgets) == 0
print("done!")
Which fails with the following exception, when it tries to query Widget at the
end:
Traceback (most recent call last):
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\ext\declarative\clsregistry.py,
line 281, in __call__
x = eval(self.arg, globals(), self._dict)
File "", line 1, in
NameError: name 'Employer' is not defined
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File multiple_bases_polymorphic_mapper_configuration_repro.py, line 62, in
widgets = session.query(Widget).all()
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\session.py,
line 1399, in query
return self._query_cls(entities, self, **kwargs)
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\query.py,
line 141, in __init__
self._set_entities(entities)
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\query.py,
line 152, in _set_entities
self._set_entity_selectables(self._entities)
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\query.py,
line 182, in _set_entity_selectables
ent.setup_entity(*d[entity])
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\query.py,
line 3668, in setup_entity
self._with_polymorphic = ext_info.with_polymorphic_mappers
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\util\langhelpers.py,
line 767, in __get__
obj.__dict__[self.__name__] = result = self.fget(obj)
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\mapper.py,
line 2011, in _with_polymorphic_mappers
configure_mappers()
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\mapper.py,
line 3029, in configure_mappers
mapper._post_configure_properties()
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\mapper.py,
line 1828, in _post_configure_properties
prop.init()
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\interfaces.py,
line 184, in init
self.do_init()
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\relationships.py,
line 1655, in do_init
self._process_dependent_arguments()
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\relationships.py,
line 1712, in _process_dependent_arguments
self.target = self.mapper.mapped_table
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\util\langhelpers.py,
line 767, in __get__
obj.__dict__[self.__name__] = result = self.fget(obj)
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\orm\relationships.py,
line 1628, in mapper
argument = self.argument()
File
python36\lib\site-packages\sqlalchemy-1.2.7-py3.6-win-amd64.egg\sqlalchemy\ext\declarative\clsregistry.py,
line 293, in __call__
(self.prop.parent, self.arg, n.args[0], self.cls)
sqlalchemy.exc.InvalidRequestError: When initializing mapper
Mapper|Mammal|mammal, expression 'Employer' failed to locate a name ("name
'Employer' is not defined"). If this is a class name, consider adding this
relationship() to the <class '__main__.Mammal'> class after both dependent
classes have been defined.
I think the problem is that Mapper._new_mappers is global, so we call
configure_mappers() even though there are no new mappers for this
declarative_base (FirstBase), so I don't think it should be necessary.
I imagine that the fix would be something like moving _new_mappers onto the
Metadata itself, but I don't know how difficult it would be. Is this something
that you would consider?
Alternatively, a simpler workaround for our case, where we do not need
polymorphism on the Widget class, would be to short-circuit
_with_polymorphic_mappers() in such cases, so that it doesn't need to call
configure_mappers at all.
Thanks, Chris.
This email is confidential. If you are not the intended recipient, please
advise us immediately and delete this message.
The registered name of Cantab- part of GAM Systematic is Cantab Capital
Partners LLP.
See - http://www.gam.com/en/Legal/Email+disclosures+EU for further information
on confidentiality, the risks of non-secure electronic communication, and
certain disclosures which we are required to make in accordance with applicable
legislation and regulations.
If you cannot access this link, please notify us by reply message and we will
send the contents to you.
GAM Holding AG and its subsidiaries (Cantab – GAM Systematic) will collect and
use information about you in the course of your interactions with us.
Full details about the data types we collect and what we use this for and your
related rights is set out in our online privacy policy at
https://www.gam.com/en/legal/privacy-policy.
Please familiarise yourself with this policy and check it from time to time for
updates as it supplements this notice.
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
---
You received this message because you are subscribed to the Google Groups
"sqlalchemy" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To post to this group, send email to [email protected].
Visit this group at https://groups.google.com/group/sqlalchemy.
For more options, visit https://groups.google.com/d/optout.
